This invention relates generally to a linearly polarizing internal mirror type gas laser tube for use in, among others, optical data processing. More particularly, this invention relates to a gas laser tube comprising a pair of holders for compressively holding a Brewster plate in the laser tube.
As will later be described more in detail with reference to one of several figures of the accompanying drawing, a gas laser tube comprises a capillary in a gas-filled space enclosed with an envelope. Laser oscillation takes place through the capillary with those mirrors used as a resonator for the laser oscillation, which are placed axially on both sides of the capillary. When the mirrors are fixed to the respective axial ends of the evelope in direct contact with the gas-filled space, the laser tube is called an internal mirror type gas laser tube. A laser beam generated axially along the capillary is taken out through a predetermined one of the mirrors as an output laser beam.
It is preferred that the gas laser tube should comprise a plate which is transparent to the laser beam and has a pair of optical flat and parallel principal surfaces. The plate is fixedly held in the gas-filled space beween the capillary and one of the mirrors with the Brewster angle formed between a common normal to the principal surfaces and the axis of the laser beam or of the capillary. The output laser beam is substantially linearly polarized. The electric vectors of the linearly polarized output laser beam are on a plane perpendicular to the principal surfaces. In other words, the plane of polarization of the output laser beam is perpendicular to the plane of incidence of the laser beam reciprocating between the mirrors. As described in U.S. Pat. No. 3,790,900 issued to Hans Golser, assignor to Siemens Aktiengesellschaft, the plate is known as a Brewster plate in the art.
The Brewster plate must be held fixedly relative to the laser beam axis. Otherwise, it is impossible to keep the output laser beam excellently linearly polarized. Furthermore, the laser oscillation becomes unstable.
An example of the linearly polarizing internal mirror type gas laser tube is revealed in U.S. Pat. No. 4,064,466 issued to Fumio Seki et al., assignors to the present assignee. According to Seki et al, the Brewster plate is held in a hollow cylindrical seal casing or external holder communicating with the gas-filled space. For this purpose, a hollow cylindrical internal holder is provided with an axial end cut to the Brewster angle. The internal holder is snugly received in the external holder with the other axial end fixedly supported by the external holder. The Brewster plate is interposed between the internal holder and another similar internal holder and is secured in position and orientation by an axial compressive force produced as a result of differences in the coefficients of thermal expansion between the external holder and members placed between the fixed axial ends of the respective internal holders.
When applied with too strong an axial compressive force, the Brewster plate is subjected to deformation. Strains brought about in the Brewster plate result in undesired production of electric vector components parallel to the Brewster plate to deteriorate the linear polarization of the output laser beam, The laser tube disclosed in the above-cited U.S. Pat. No. 4,064,466 is excellent in this respect because the internal holders are brought into contact with the Brewster plate only at four corners thereof. The internal holders are, however, expensive because it has been necessary to manufacture each internal holder for a metal pipe or from a glass pipe. Moreover, it has been troublesome to evacuate very narrow spaces formed between the external holder and the respective internal holders.